1. Field of the Invention
This invention is concerned with an improvement in the production of formaldehyde from methanol. In particular, it relates to such a process where use of small amounts of phosphorus containing compounds improve selectivity to formaldehyde.
2. Prior Art
The catalytic conversion of methanol to produce formaldehyde has been known since 1878. The use of silver catalysts for such a process has been known since at least 1908 and is disclosed in German Patent No. 228,687. Other metals or metal alloys as well as metal oxides have also been suggested and used for this process.
Commercially there are two processes that are in widespread use. The first utilizes silver as the catalyst. This process is carried out in a methanol rich atmosphere. The second uses a metal oxide catalyst and operates in an oxygen rich atmosphere. Basically the first process is operated at approximately one to three atmospheres absolute, although other pressure ranges can be employed if desired. Methanol and air are passed over a stationary bed of the catalyst. The overall reaction is exothermic in nature and can operate anywhere between 450.degree.-750.degree. C. depending upon process and product requirements. The mixture which is passed through the catalyst bed is not restricted to methanol and air only since various diluents have been disclosed in the literature. Diluents may consist of steam, carbon oxides, and recycled off gases, including formaldehyde.
The mechanism of formaldehyde production is believed to be a combination of two reactions, namely the dehydrogenation and oxidative dehydrogenation of methanol. EQU CH.sub.3 OH.fwdarw.CH.sub.2 O+H.sub.2 EQU CH.sub.3 OH+1/2O.sub.2 .fwdarw.CH.sub.2 O+H.sub.2 O
Depending upon the conversion required, the silver catalyzed process can be operated in stages. Single stage operation allows only moderate amounts of methanol to be converted in a single pass limited by reaction temperature unless a heat sink such as inerts or water is employed. This necessitates distillation of unreacted methanol from the product formaldehyde and entails substantial investment in distillation facilities.
Another way to operate the process which eliminates distillation or other concentration facilities is to use two or more catalytic stages with interstage cooling. A basic two stage process is disclosed in U.S. Pat. No. 2,462,413 to Meath. An improvement over the Meath process where even lower amounts of methanol in the product can be obtained is disclosed in U.S. Pat. No. 3,959,383 issued to Northheimer.
The use of substantial amounts of trisubstituted organic phosphates and small, but an undetermined amount of phosphine in the production of ethylene oxide from ethylene and air at operating temperatures of .about.310.degree. C. using a supported silver catalyst is disclosed in U.S. Pat. No. 2,587,468 issued to R. L. Heider.